Only humans and dogs are capable of understanding finger-pointing (Image: The Evening Standard/Hulton Getty)

Our brains have shrunk in the past 20,000 years. In The Domesticated Brain, Bruce Hood argues that it's a result of living together in societies

TWENTY thousand years ago, the average human brain was 10 per cent larger than it is today. Some people, such as David Geary, a psychologist at the University of Missouri in Columbia, claim that the dip in cranial capacity marks our dwindling intelligence. Others, like John Hawks, an anthropologist at the University of Wisconsin-Madison, attribute it to improved brain efficiency.

But for Bruce Hood, the author of The Domesticated Brain and a psychologist at the University of Bristol, UK, the shrinkage is best explained by changes in society. "We have been self-domesticating through the invention of culture and practices that ensure that we can live together," he writes. Our brains, he believes, are getting downsized by domesticity.

Domestication tends to have that effect. According to Hood, every species that has been domesticated by humans has lost brain capacity as a result. Bred for passivity, their testosterone decreases, reducing the size of all organs. Dogs are a good example and the effect on their behaviour is telling: where wolves will try to solve a problem through cunning, dogs are adept at soliciting help from their masters.

Drawing on his research in developmental psychology, Hood often enlists parallels between dogs and children to support the notion of human domestication. Like dogs, kids are highly skilled at enlisting assistance. Even infants have the knack, getting parents to fetch an out-of-reach object with a glance. Also like dogs, they are great readers of social cues: only dogs and humans know to follow a pointed finger to an object.

Of course, human culture is more sophisticated than the domestication of dogs, and Hood is highly attentive to differences between humans and other creatures. Imitation is one interesting area of distinction. Chimps and pre-school children both mimic the actions of others in order to learn a new skill. But a chimp will imitate only the motions necessary to achieve the goal, whereas a child also mimics steps clearly unrelated to the task. "Why would children over-imitate a pointless action?" asks Hood. Because they are more interested in fitting in than in learning how best to solve the task, he says.

Hood argues that our social adeptness is both a cause and an effect of our self-domestication, and suggests that our social behaviour is key to our species' success. Knowledge can be broadly distributed, disparate areas of expertise collaboratively coordinated, and technology can develop over many generations.

Hood also acknowledges that our socially domesticated brains are responsible for prejudice, and can condone horrific acts, such as genocide. The importance we place on allegiances, for example, is all too easily manipulated by unscrupulous leaders, and deplorable actions are too readily committed through what Hood calls "diffusion of accountability".

Understanding the good or bad consequences of domestication is invaluable to us because the self-reflexiveness that made us who we are can also, potentially, improve us in the future. For that important reason, Hood is to be commended for writing The Domesticated Brain at a level that anyone can understand.

That said, in his effort to encompass all of psychology in just 300 pages – evidently the remit of a Pelican Introduction title – he often loses touch with his theme. The result is informative but, sadly, largely formless.

This article appeared in print under the headline "Where did our brains go?"

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